I have been trying to narrow down a SQL Server connection timeout problem, that only occurs when a database is mirrored. I have yet to be able to really figure out the cause, but here is the code that describes the problem.

[TestFixture]
public class SqlMirroringTests
{
    const string connectionString = "Data Source=some_server;Initial Catalog=some_db; User ID=*****;Password=*****;";
    private static Random rand = new Random();
 
    private ArrayList times = ArrayList.Synchronized(new ArrayList());
    private ArrayList erroredTimes = ArrayList.Synchronized(new ArrayList());
 
    private void DbScenario(int identifier)
    {
        var sleepTime = Random();
 
        WriteInfo(identifier, "Sleeping " + sleepTime + "ms");
        Thread.Sleep(sleepTime);
 
        using (var conn = new SqlConnection(connectionString))
        {
            var stopWatch = new Stopwatch();
            stopWatch.Start();
 
            bool isErrored = false;
            try
            {
                conn.Open();
            }
            catch(Exception e)
            {
                Console.WriteLine(e);
                isErrored = true;
                throw e;
            }
            finally
            {
                var timespan = stopWatch.Elapsed;
                WriteInfo(identifier, "Open time: " + timespan);
 
                times.Add(timespan);
 
                if (isErrored)
                {
                    erroredTimes.Add(timespan);
                }
            }
 
            //We have to actually query for something to get the error, and it needs to be a decent chunk of data
            var command = conn.CreateCommand();
            command.CommandText = "select * from <Insert any table here with some data>";
            command.ExecuteReader();
 
            conn.Close();
        }
        WriteInfo(identifier, "DONE");
    }
 
    [Test]
    public void SlamDatabase()
    {
        var exceptions = new List<Exception>();
        var action = new Action<int>(DbScenario);
 
        var results = Enumerable.Range(0, 250)
            .Select(i => action.BeginInvoke(i, null, null))
            .ToList();
 
        results.ForEach(ar => {
            try
            {
                action.EndInvoke(ar);
            }
            catch (Exception e)
            {
                exceptions.Add(e);
            }
        });
 
        Console.WriteLine("Number of Exceptions: " + exceptions.Count);
        WriteTimeResults("All times - ", times);
        WriteTimeResults("Error times - ", erroredTimes);
    }
 
    private void WriteTimeResults(string prefix, ArrayList results)
    {
        Console.WriteLine(prefix + "Max Open time: " + results.Cast<TimeSpan>().Max());
        Console.WriteLine(prefix + "Avg Open time: " + results.Cast<TimeSpan>().Average(t => t.TotalSeconds));
    }
 
    private int Random()
    {
        return rand.Next(1000);
    }
 
    private void WriteInfo(object state, string message)
    {
        Console.WriteLine("Thread " + state.ToString() + " - " + message);
    }
}

It throws two possible errors:

System.Data.SqlClient.SqlException: Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.
   at System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject)
   at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
   at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
   at System.Data.SqlClient.SqlConnection.Open()
   at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
   at System.Data.SqlClient.SqlConnection.Open()
   at System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject)
   at System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject)
   at System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject)
   at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
   at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
   at System.Data.SqlClient.SqlConnection.Open()

And …

System.Data.SqlClient.SqlException: Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.
   at System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject)
   at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
   at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
   at System.Data.SqlClient.SqlConnection.Open()

The odd thing is. Here are the results:

Number of Exceptions: 118
All times - Max Open time: 00:00:02.7301659
All times - Avg Open time: 0.0934956888
Error times - Max Open time: 00:00:02.7301659
Error times - Avg Open time: 0.0855163728813559

118 errors out of 250! After one happens, a ton of others happen. I am guessing the pool get’s jacked up and starts looking at the mirror to login temporarily or something, but I can’t figure out why. Increasing the Connect Timeout resolves the problem, but I HATE increasing timeouts. The funny thing is, according to my results, the average timed out connection is .08s. That 625x less than the connect timeout.

Finally, to make matters even more confusing, if you look at my connection string, I didn’t setup the connection for failover. Although I believe that failover still works, except on the initial connection.

Any ideas would be greatly appreciated.